As the chip manufacturing process progresses towards making smaller and finer chip circuitry, the different leakage currents within the chip, including the subthreshold leakage current, gate tunneling leakage current and GIDL (Gate-Induced Drain Leakage) current increase as described in “Identifying defects in deep-submicron CMOS ICs”, IEEE Spectrum, pp. 66-71, September, 1996 (hereafter referred to as reference 1). These leakage currents increase the electrical current consumption of the chip.
A method for reducing subthreshold leakage current in the related art is disclosed in “A Low Power Data Holding Circuit with an Intermittent Power Supply Scheme”, Symposium on VLSI Circuits Digest of Technical Papers, pp. 14-15, 1996 (hereafter referred to as reference 2). In the method of reference 2, a power switch composed of MOS transistors having a threshold voltage with an absolute value sufficiently higher than the absolute values of threshold voltages of MOS transistors comprising the circuit block, is inserted in series with the circuit block power supply between power and ground. Here, the term MOS transistor is used in these specifications as a general term to describe an insulated gate type field effect transistor. Also, the power supply voltage supplied to the circuit is defined as having a high voltage (potential) and low voltage (potential). However, this is used here to respectively express the power supply as the high voltage potential and the ground as the low voltage potential. While the chip is in standby (idle), the subthreshold leakage current flowing through the circuit block is cut off by turning this power switch off. Usually, setting this power switch to off, erases the information stored in the information retention circuits (in circuits with a volatile information holding function for example; static memories, flip-flops, latches and register files, etc.) within the circuit block because the supply of power to the circuit blocks is cut off. In fact however, a particular time (TR) is required before the information within the information retention circuits is erased after setting the power switch to off. The method in reference 2 therefore turns the power switch on once again before the time TR elapses after the power switch was turned off (Hereafter, the operation of turning the power supply on once again is called the refresh operation.) Then, the power supply switch is turned off after a fixed amount of time, and this process is repeated to prevent information within the information retention circuit from being erased and to reduce the amount of current consumption in the circuit block due to subthreshold leakage current.
A method disclosed in “A Novel Powering-down Scheme for Low Vt CMOS Circuits”, Symposium on VLSI Circuits Digest of Technical Papers, pp. 44-45, 1998 (hereafter referred to as reference 3) has the same power switch and circuit block connections as in reference 2. In the method of reference 3, a diode is connected in series with the power switch to clamp (reduce) to a lower level the excess voltage (voltage differential between power and ground) supplied to the circuit block while the power switch is on and prevent the loss of information from information retention circuits in the circuit block. In the figures given in the example in reference 3, the voltage differential of the power supplied to the circuit block while the power switch is off is 0.7 or more volts and is the threshold voltage (PMOS is −0.14 volts and NMOS is 0.31 volts) of the MOS transistor comprising the circuit block.